On-chip lasers that exhibit efficiency, low noise, stability are useful for a number of important applications ranging from integrated analog photonics and microwave generation to coherent communications and laser detection and ranging (LADAR). Integrated lasers can be realized via Germanium-on-Silicon heterojunctions, hybrid integration with III-V semiconductor materials, stimulated Raman scattering, and erbium-doped glass on silicon. Unfortunately, germanium lasers exhibit large threshold currents, relatively low internal quality factors, and broad spectral linewidth. Despite demonstrating high efficiency lasing with electrical pumping, III-V semiconductor heterojunction lasers tend to exhibit broad linewidth and corresponding high phase noise levels due to their limited internal quality factors and large thermo-optic coefficients. Moreover, integration of III-V chips or wafers to silicon is a complicated fabrication process that can lead to low yields.
Erbium-doped glass lasers can be made using a straightforward, monolithic fabrication process that yields high-performance, narrow-linewidth lasers. In particular, erbium-doped aluminum oxide (Al2O3:Er3+) has been co-sputtered onto oxidized silicon wafers with relatively low loss and a broad gain spectrum to form racetrack and ultra-narrow-linewidth distributed feedback (DFB) lasers. But the laser waveguides and cavities in previous erbium-doped glass lasers have been made using interference lithography and by etching the gain material, both of which are difficult to incorporate within standard wafer-scale silicon photonics process flows.
It is also difficult to fabricate erbium-doped glass DFB lasers with phase-shifted gratings using interference lithography and gain material etching. Typically, laser diodes have integrated quarter-wave phase-shifted Bragg gratings to ensure single wavelength lasing for both long-haul fiber-optic telecommunications and short-reach on-chip data communications. Furthermore, in the DFB laser arrays used for wavelength division multiplexing (WDM), the DFB lasers in the array have gratings that are phase-shifted by precise amounts to ensure that their output wavelengths are aligned with the channels on the WDM wavelength grid. In the telecommunications C-band, these channels are normally several nanometers apart, which corresponds to picometer-scale variations in the grating spacings. Unfortunately, picometer-scale variations are difficult to achieve using photolithography.